mips64: add support for load indexed instructions from DSP ASE

Handling lwx, ldx and lbux for MIPS-Cavium processors.

Patch by Zahid Anwar, with some changes.

Related to Bugzilla issue 326444.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2819 8f6e269a-dfd6-0310-a8e1-e2731360e62c